Trust is an asset in web-based interactions. For example, a user must trust that an entity provides sufficient mechanisms to confirm and protect her identity or other confidential information in order for the user to feel comfortable interacting with such entity. Further, an entity that provides a web-resource must be able to block automated attacks that attempt to gain access to the web-resource for malicious purposes. Thus, sophisticated authentication mechanisms that can discern between a resource request originating from a human being and a request generated by an automated machine are a vital tool in ensuring that web-resources are protected from automated attacks and developing the necessary relationship of trust between a resource provider and a user.
CAPTCHA systems (“completely automated public Turing test to tell computers and humans apart”) can provide such an authentication mechanism. One goal of a CAPTCHA system is to exploit situations in which it is known that humans perform tasks better than automated machines. Thus, as part of a verification process, CAPTCHA systems can provide a challenge that is solvable by a human but generally unsolvable by a machine.